This invention relates generally to powder coatings and, more particularly, to the continuous processing of powder coatings.
Because of increased environmental concerns, much effort has been directed to the problem of reducing pollution caused by the evaporation of solvents from paints. These efforts have led to the development of new coating technologies which eliminate or at least diminish the emission of organic solvent vapors into the atmosphere. Since the mid-1950""s, the powder coating technology has been one of the most successful developments in terms of reducing or eliminating solvent vapor emissions.
The use of powder coating compositions can be extremely desirable as such compositions are essentially free of organic solvents such as are conventionally present in liquid paint systems. Accordingly, economic and social benefits such as reductions in air pollution, energy requirements, and fire and health hazards can be realized through the use of powder coatings.
A common technique for applying a powder coating to an object makes use of electrostatic powder spray coating equipment. In such application, a coating powder is dispersed in an airstream and passed through a high voltage field whereby the coating particles attain an electrostatic charge. These charged particles are attracted to and deposited on the object to be coated which is usually at room temperature. Subsequently, the object is placed in an oven and heated whereby the powder melts/cures to form the desired coating on the object.
U.S. Pat. No. 5,009,367 to Nielsen, U.S. Pat. No. 5,027,742 to Lee et al., and xe2x80x9cHIGHER SOLIDS COATINGS ABOVE 80% BY VOLUME,xe2x80x9d presented at the Water-Borne and Higher Solids Coatings Symposium, Mar. 10-12, 1980, all concern the spraying of materials using supercritical fluids.
Further, based on U.S. Pat. No. 5,158,986 to Cha et al., U.S. Pat. No. 5,334,356 to Baldwin et al., and the article entitled, xe2x80x9cNEW ROLES FOR SUPERCRITICAL FLUIDS,xe2x80x9d appearing in Chemical Engineering, March 1994 (pages 32-35), it is known to feed fluids, including supercritical CO, to an extruder to form an extruded shape of a fluid and polymer plastic material. As disclosed, such extruded material can subsequently be processed to form a desired supermicrocellular, foamed material, such as in the form of a sheet.
Conventionally, the manufacture of a powder coating comprises melt-mixing a resin, a curing agent, plasticizers, stabilizers, flow aids, pigments, and extenders. Whereas dry blending is commonly used to make PVC powders under conditions not amenable to the formation of very fine powders, melt-mixing involves the high speed, high intensity mixing of dry ingredients in a Henschel mixer or the like and then the heating of the mixture to an elevated temperature (e.g., about 180-250xc2x0 F.) in a continuous compounder such as a single or twin screw extruder to achieve thorough dispersion of the other ingredients in the resin as the resin melts, forming a molten mixture. The molten mixture is then cooled to quench the reaction and crushed. Such processing is then generally followed by a sequence of operations which can involve grinding, sifting, separation, and filtering, followed by more sieving.
Such manufacture and processing of coating powders, however, are subject to a number of shortcomings or difficulties. For example, high temperature processing of ingredients in a melt extruder can bring about premature reaction of the resin with the curing agent or degradation of at least some polymer resins.
Additionally, the particles produced as a result of such crushing and grinding operations are generally substantially non-spherical, irregularly shaped. Such irregularly shaped particles can have an undesirable effect on the uniformity and continuity of any resulting coating formed on a substrate surface as a result of application and curing of such a powder coating.
Furthermore, the particles produced by such conventional manufacture processing tend to vary greatly in size. Consequently, various particle separation techniques such as screening and cyclone separation can be required in order to separate undesirable large and small particles from the powder particles having the desired size distribution. The powder particles which are undesirably sized must then typically be downgraded or otherwise disposed of.
In the past, various approaches have been proposed in order to overcome or minimize some of the above-identified problems.
U.S. Pat. No. 5,207,954 to Lewis et al. discloses a method of making a thermosettable, coreactable particulate powdered composition of a first copolymer of an olefinically unsaturated monomer having at least one functional group and at least a second copolymer of an olefinically unsaturated monomer having at least one functional group which is reactive with the functional group of the first copolymer. Aqueous dispersions containing the coreactive polymers are disclosed as being spray dried to produce copolymeric particles which are substantially uniform and spherical in shape.
U.S. Pat. Nos. 4,582,731 and 4,734,451, both to Smith, disclose methods and apparatus for the deposition of thin films and the formation of powder coatings through the molecular spray of solutes dissolved in organic and supercritical fluid solvents. The examples disclose the application of single component films to substrate surfaces. These patents do not appear to disclose coating materials composed of multiple components or materials, or the processing thereof.
U.S. Pat. No. 5,290,827 to Shine concerns a process for preparing a homogeneous blend of otherwise thermodynamically immiscible polymers, rather than resins with or without a curing agent. In accordance with the disclosure, mixtures of polymers are dissolved under pressure in supercritical fluid solvents and then expanded through a fine nozzle. As the supercritical fluid solvent evaporates, the polymer mixture is disclosed as depositing as a substantially homogeneous blend.
U.S. Pat. No. 5,399,597 to Mandel et al. discloses a batch process for preparing powder coating materials whereby at least some of the above-identified problems are sought to be minimized or avoided. In accordance with the process thereof, different first and second organic materials and a supercritical fluid are mechanically agitated in a first container. The contents of the first container are then discharged into a second container, maintained at a lower pressure than the first container, and in which substantially all of the first and second organic materials are collected.
Such batch processing can suffer from a number of shortcomings. For example, batch processing can undesirably result in long cycle times which, for example, can cause undesired polymerization of fast curing powder coating compositions. Further, batch processing can lead to product inconsistencies, such as inconsistencies in product properties such as viscosity and particle size, due to variations in processing conditions such as pressure and mixing time over the course of a batch run. Still further, large batch runs will typically necessitate the use large processing vessels. Large processing vessels can in turn prove undesirably time consuming to properly clean between runs for or with different product compositions. In addition, in such batch processing it can be difficult to maintain high pressure seals such as typically required to contain supercritical process fluids.
Further, U.S. Pat. No. 5,399,597 emphasizes that with the process disclosed therein, solubilization of components in the supercritical fluid is undesirable as such solubilization would unavoidably result in lose of material upon transfer from the process vessel to the product receiving vessel. The patent teaches the avoidance of such undesirable results through the selection of materials which are not soluble in the supercritical fluid at the operating conditions.
A general object of the invention is to provide improved processing of powder coatings.
A more specific objective of the invention is to overcome one or more of the problems described above.
The general object of the invention can be attained, at least in part, through the production of a powder coating by a method wherein a stream of a powder coating precursor is contacted with a process media fluid effective to reduce the viscosity of the powder coating precursor stream to allow processing of the powder coating precursor stream at a lower temperature. The powder coating precursor stream includes powder coating ingredients including at least one resin and at least one additional powder coating ingredient. The process media fluid includes a process media material in the form of a fluid selected from the group consisting of supercritical fluids, gases, and liquified gases.
In one particular embodiment, the process media fluid is effective to plasticize at least one of the resin and additional powder coating ingredient.
In another particular embodiment, the process media fluid is a supercritical fluid effective to wholly or partially dissolve at least one of the resin and additional powder coating ingredient.
In yet another particular embodiment, the process media fluid is a gas effective to dilute the mixture of powder coating ingredients, thereby reducing the viscosity of the mixture in the extruder.
The prior art fails to provide systems, apparatus combinations and methods for continuous process production of powder coatings, particularly the production of powder coatings having greater uniformity in one or more properties or characteristic such as particle size, shape, color, gloss and cure rate.
The invention further comprehends a method for producing a powder coating wherein powder coating raw materials are fed to and processed in a continuous extruder. The powder coating raw materials fed to and processed in the continuous extruder include at least one resin and at least one additional powder coating ingredient, with the extruder processing being effective to disperse the at least one additional ingredient with the at least one resin to form an extrudate product. The method of the invention includes the step of adding a process media fluid comprising a process media material in the form of a fluid selected from the group consisting of supercritical fluids, gases, and liquified gases to a process stream of at least one of the following:
a.) raw materials fed to the continuous extruder;
b.) raw materials processed in the continuous extruder; and
c.) the extrudate product of the continuous extruder, the addition of the process media fluid being effective to reduce the viscosity of the selected process stream to allow processing of the process stream at a lower temperature.
The invention still further comprehends a method for producing a powder coating wherein a premixed blend of powder coating raw materials are extruded to form an extrudate product. In one embodiment, the premixed blend of powder coating raw materials includes at least one thermosettable resin and at least one curing agent for the at least one thermosettable resin. A stream of the extrudate product is then fed through a melt pump to form a stream of extrudate product at increased pressure. The stream of extrudate product at increased pressure is then spray dried to form the powder coating.
In accordance with this method, at least one of the blend of powder coating raw materials undergoing extrusion and the stream of extrudate product at increased pressure is contacted with a process media fluid selected from the group consisting of supercritical fluids, gases, and liquified gases. The process media is effective to reduce the viscosity of the materials of the selected process stream to allow processing at a lower temperature.
The invention also comprehends systems for producing a powder coating. In accordance with one embodiment, the powder coating producing system of the invention includes a continuous extruder wherein powder coating raw materials including at least one resin and at least one additional powder coating ingredient are fed and processed to disperse the at least one additional ingredient with the at least one resin to form an extruded coating precursor stream.
The system also includes a source of a process media material. The process media material achieves a fluid condition within the processing system and is effective to reduce the viscosity of the powder coating precursor stream to allow processing of the powder coating precursor stream at a lower temperature.
The system further includes means for adding such process media material to at least one of the following:
a.) raw materials fed to the continuous extruder;
b.) raw materials processed in the continuous extruder;
c.) the extruded product of the continuous extruder, and means for forming and separating the coating powder from the process media material.
In particular embodiments, such means for forming and separating the powder coating from the process media material can take various forms including: spray drying (including spray drying into a reclamation booth), formation of a foam or friable mass suitable for subsequent grinding or the like reduction into a desired powder form, and spraying into a solution.
The invention also comprehends a system for producing a coating powder including a continuous extruder, a melt pump, a source of a process media material. In the continuous extruder, powder coating raw materials including at least one thermosettable resin and at least one curing agent for the at least one thermosettable resin are fed and processed to disperse the at least one curing agent with the at least one thermosettable resin to form a molten extruded coating powder precursor. The molten extruded coating powder precursor is processed through the melt pump to form a powder coating precursor stream of increased pressure. The source of a process media material contains a process media material which, within the processing system, is in a fluid condition and effective to reduce the viscosity of the powder coating precursor stream to allow processing of the powder coating precursor stream at a lower temperature.
The system further includes means for adding such process media material to the powder coating raw materials in the continuous extruder and a spray drier to form and separate the powder coating from the process media material.
As used herein, references to a xe2x80x9csupercritical fluidxe2x80x9d are to be understood to refer to a material that is at a temperature and pressure such that it is at, above, or slightly below its critical point.
As used herein, the xe2x80x9ccritical pointxe2x80x9d is the transition point at which the liquid and gaseous states merge into each other and represents the combination of the critical temperature and critical pressure for a given substance.
The xe2x80x9ccritical temperature,xe2x80x9d as used herein, is defined as the temperature above which a gas cannot be liquefied by an increase in pressure.
The xe2x80x9ccritical pressure,xe2x80x9d as used herein, is defined as that pressure which is just sufficient to cause the appearance of two phases at the critical temperature.
As used herein, references to a xe2x80x9cliquified gasxe2x80x9d are to be understood to refer to a material which is a liquid but which at standard conditions of temperature and pressure is in the gaseous state.
As used herein, references to a gas or xe2x80x9cgasesxe2x80x9d are to be understood to refer to a material which is in the gaseous state at the standard temperature and pressure of 0xc2x0 C. and 760 mm and under the conditions of temperature and pressure employed in the process of this invention. It is also to be understood that a liquified gas may be admixed with a gas as defined herein.
The terms xe2x80x9cpowder coatingxe2x80x9d and xe2x80x9ccoating powderxe2x80x9d are used interchangeably to mean the powder from which a coating is to be made, unless the context requires that xe2x80x9cpowder coatingxe2x80x9d means the coating, itself.
The term xe2x80x9cgenerally spherical particles,xe2x80x9d as used in the context of this invention, encompasses particles having true spherical shapes to those having near spherical shapes. Near spherical shapes include ovoid shaped particles; particles having open or closed bulbous protuberances, such protuberances may or may not be generally spherically shaped; and particles having cellular portions therein. Such cellular portions may extend or be contained internally and/or externally of the major surface of the particle and may be open or closed.
The term xe2x80x9ccellular,xe2x80x9d as used in the context of this invention, means having at least some hollow portion or portions.